A Batch of Letter Sized Airmail Envelopes from a County

A batch of letter sized airmail envelopes from a county well known as a source of narcotics,that were bulky and much heavier than ordinary airmail letters,are stopped at the Post Office in New York.A U.S.Customs Inspector opens the letters and finds heroin.No search warrant was sought.Would this be considered an illegal search and seizure?

A) Yes,because there was no probable cause to link the envelopes to any specific crime;smuggling or not.
B) No,because it constitutes a "border search" and anything crossing the border falls within this exception.
C) No,but only because "probable cause" was not articulated nor was a search warrant obtained.
D) No,it would only be admissible with a warrant or with consent,regardless of the border issue,since it involved the U.S.Mail.